Lecher wire system



Nov. 20, 1945. c. N. SMYTH 2,389,167

' LECHER WIRE SYSTEM Filed June 25, 1941 1 Invmm A Home Patented Nov. 20, 1945 LECHER WIRE SYSTEM Charles Norman Smyth, London, England, as-

signor to International poration, New York, N.

Application June 25, 1941, Serial No. 399,667 In Great Britain June 21, 1940 2 Claims.

In order to prevent a pair of lecher wires which are screened by an enclosing tube from resonat- 3*;- asymmetrically as a co-axial line, rather than symmetrically as a. balanced pair, a resistance equal to the characteristic impedance of the arrangement as a co-axial line is in accordance with the present invention connected at one end between the lecher wires and the screening tube.

In a broader aspect the invention resides in a screened lecher wire system characterised by a dissipative impedance in a symmetrical connection between the lecher wires and the screen.

Lecher wires are often used in the input circuits of sensitive wireless receivers for ultra high frequencies, and in certain cases it is most essential that they should be accurately balanced, e. g. in the "construction of balanced frequency changers.

The balance will be disturbed if these lecher wires, which must be screened, are able to resonate as a co-axial line.

In order to reduce the possibility of this, the concentric line circuit is terminated by its characteristic impedance, thereby preventing the formation of any standing waves. In some cases the amplitude of the standing waves in the coaxial line may be suficiently reduced by a resistance differing from .the characteristic imv pedance. I

An arrangement embodying the invention is shown in the accompanyin drawing, Fig. 1 being a longitudinal section and Fig. 2 a section on line XX of Fig. 1.

A and A are a pair of lecher wires inside a screen B. They are coupled to an aerial by means of the loop C and the ends D and D of the lecher wires feed to a balanced modulator.

The resistance E is connected between the ISrtandard Electric Cor- The value of this resistance to equal the characteristic impedance of the co-axial system can readily be calculated from the formula:

where and 2h being the distance between the centres of the lecher wires, 0 the inner radius of the screen and b the radius of the lecher wires, and the logarithms being evaluated to the base Ill.

What is claimed is: Y

1. A lecher wire system comprising a pair of spaced wires enclosed by and extending along a screening tube wherein the transmission line constituted by the tube as one conductor and an impedance bridged between said wires and said screening tube, said impedance being of such value that said transmission line and the pair of wires as the other conductor is terminated in an impedance substantially equal to its characteristic impedance.

2. A lecher wire system comprising spaced parallel wires bridged at one end and coupled to a utilisation circuit at the other end, a tubular screen enclosing the wires said wires and screen short-circuited end of the lecher wire pair and the screen.

having the characteristics of a coaxial line and a resistance adjusted to provide an impedance matching termination for said coaxial line connected between the bridged ends of the wires and an adjacent part of the screen.

1 CHARLES NORMAN SMY'I'H.

Certificate of Correction Patent No. 2,389,167. November 20, 1945.

CHARLES NORMAN SMYTH It is hereby certified that error appears in the printed specification of the above numbered oatent requiring correction as ioiiows: Page 1, second column, lines 24 to 27 lHCiUSWG, claim 1, strike out and an impedance bridged between sald wiresand said screening tube, said impedance being of such value that said transmission line and insert the same in line 28, same claim, after conductor; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 30th day of September, A. D. 1947.

[our] THOMAS F.v MURPHY,

Assistant commissioner of Patents. 

